Modulation agnostic digital hybrid mode power amplifier system and method

ABSTRACT

A RF-digital hybrid mode power amplifier system for achieving high efficiency and high linearity in wideband communication systems is disclosed. The present invention is based on the method of adaptive digital predistortion to linearize a power amplifier in the RF domain. The present disclosure enables a power amplifier system to be field reconfigurable and support multi-modulation schemes (modulation agnostic), multi-carriers and multi-channels. As a result, the digital hybrid mode power amplifier system is particularly suitable for wireless transmission systems, such as base-stations, repeaters, and indoor signal coverage systems, where baseband I-Q signal information is not readily available.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/928,931, filed Dec. 21, 2010; which claims priority to U.S.Provisional Patent Application No. 61/288,844, filed on Dec. 21, 2009.The disclosures of each are hereby incorporated by reference in theirentirety for all purposes.

FIELD OF THE INVENTION

The present invention generally relates to wireless communicationsystems using complex modulation techniques. More specifically, thepresent invention relates to power amplifier systems for wirelesscommunications.

BACKGROUND OF THE INVENTION

A wideband mobile communication system using complex modulationtechniques, such as wideband code division multiple access (WCDMA) andorthogonal frequency division multiplexing (OFDM), has largepeak-to-average power ratio (PAPR) specifications and hence requireshighly linear power amplifiers for its RF transmissions. Theconventional feedforward linear power amplifier (FFLPA) has been widelyutilized due to its excellent linearity performance in spite of poorpower efficiency.

Conventional FFLPAs are mainly based on the principle of errorsubtraction and power-matching with dedicated hardware circuitries torealize nonlinear corrections to PA. These approaches must use anauxiliary PA and complicated hardware circuitries to match exactly thetransmitted power-balance, time-delay and errors generated by the mainPA. After a perfect matching is obtained, the non-linear distortionerrors from the main PA can then be canceled by those distortion errorsfrom the auxiliary PA. Due to the complexities of the nonlinearpredistortion circuits, which among other things involve many variablesand parameters, FFLPAs require significant fine tuning and othercalibration efforts. In addition, such traditional FFLPA schemes arealso vulnerable to fluctuating environmental conditions, such astemperature and humidity changes, since perfect alignment of the mainPA's signal and that of the auxiliary PA are vital. As a result,traditional predistortion schemes are costly to implement and arelimited in their predistortion accuracy and stability in a commercialwireless system environment.

In order to overcome the FFLPA's poor efficiency, digital basebandpredistortion (PD) has been demonstrated due to the recent advances indigital signal processing (DSP) technology. In addition, Doherty poweramplifiers (DPA) have also been applied to these linearization systemsto improve power efficiency. However, there is still a demand for higherperformance of the power amplifier such as more linearity and betterefficiency with less expensive architecture.

Conventional DSP-based PD schemes utilize digital microprocessors tocompute, calculate and correct the PA's nonlinearities, typically byperforming fast tracking and adjustments of signals in the PA system.However, conventional DSP-based PD schemes are challenged by variationsof the linearity performance of the amplifier due to changes in theenvironment such as temperature and the asymmetric distortions of theoutput signal of the PA resulting from memory effects. All of thesevariations and distortions have to be compensated for. Conventional PDalgorithms are based on a wideband feedback signal, and require a highspeed analog-to-digital converter (ADC) in order to capture thenecessary information. In addition, time-synchronizations are typicallyrequired to capture an error signal between a reference signal and adistorted signal. This time-matching process may result in smallsynchronization errors which can further affect conventional PD schemes'linearization performance. Amplitude and phase synchronization is alsorequired in order to align the reference signal and the distortedsignal.

Moreover, conventional PD schemes necessitate coded in-phase (I) andquadrature (Q) channel signals in the baseband as the required ideal orreference signals. As a result, conventional PD schemes are oftenstandard or modulation specific and must be closely tailored to eachbaseband system. Therefore, in order to deploy conventional PD schemesinto base-stations, the PD engines must be embedded into the basebandarchitecture of base-stations. This embedment is a practicalimplementation challenge since it is frequently inconvenient orimpossible to modify the baseband architectures of existingbase-stations or base-station designs. Once the PD scheme is set up fora specific base-station design, it is often not reconfigurable and hencenot upgradeable to future changes in standards or modulations.Furthermore, since traditional PD approaches require baseband I-Q signalsources to operate, they are inapplicable to certain RF systems that donot possess any baseband I-Q signal sources, such as repeater and indoorsignal coverage sub-systems.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide a highperformance and cost effective method of power amplifier systems withhigh linearity and high efficiency for wideband communication systemapplications. The present disclosure provides a field-reconfigurablepower amplifier system that supports multi-modulation schemes(modulation agnostic), multi-carriers and multi-channels. Inmulti-channel configurations of the present invention there can be morethan one PA for multiple bands.

To achieve the above objects, the present invention is generally basedon the method of adaptive digital predistortion to linearize a poweramplifier in the RF domain. Various embodiments of the invention aredisclosed. In an embodiment, the combination of crest factor reduction,PD, power efficiency boosting techniques as well as coefficient adaptivealgorithms are utilized within a PA system. In another embodiment,analog quadrature modulator compensation structure is also utilized toenhance performance.

Some embodiments of the present invention are able to monitor thefluctuation of the power amplifier characteristics and to self-adjust bymeans of a self-adaptation algorithm. One such self-adaptation algorithmpresently disclosed is called a digital predistortion algorithm, whichis implemented in the digital domain.

Applications of the present invention are suitable for use with allwireless base-stations, access points, mobile equipment and wirelessterminals, portable wireless devices, and other wireless communicationsystems such as microwave and satellite communications.

A RF-digital hybrid mode power amplifier system for achieving highefficiency and high linearity in wideband communication systems isdisclosed. The present invention is based on the method of adaptivedigital predistortion to linearize a power amplifier in the RF domain.The power amplifier characteristics such as variation of linearity andasymmetric distortion of the amplifier output signal are sampled in afeedback path and controlled by the adaptation algorithm in a digitalmodule. Therefore, in an embodiment, the present invention is capable ofcompensating for the nonlinearities as well as memory effects of poweramplifier systems and also improves performance, in terms of power addedefficiency, adjacent channel leakage ratio (ACLR) and peak-to-averagepower ratio. The present disclosure enables a power amplifier system tobe field reconfigurable and support multi-modulation schemes (modulationagnostic), multi-carriers and multi-channels. As a result, the digitalhybrid mode power amplifier system is particularly suitable for wirelesstransmission systems, such as base-stations, repeaters, and indoorsignal coverage systems, where baseband I-Q signal information is notreadily available.

THE FIGURES

Further objects and advantages of the present invention can be morefully understood from the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a block diagram showing the basic form of a digital hybridmode power amplifier system.

FIG. 2 is a block diagram showing a simple digital predistortion blockdiagram for a power amplifier system according to one embodiment of thepresent invention.

FIG. 3 is a block diagram showing polynomial-based predistortion in adigital hybrid mode power amplifier system of the present invention.

FIG. 4 is a block diagram of the digital predistortion algorithm appliedfor self-adaptation in a digital hybrid mode power amplifier system ofthe present invention.

FIG. 5 is a delay estimation block diagram of the present invention.

FIG. 6 is a block diagram of a fractional delay for the presentinvention.

FIG. 7 is a block diagram showing a digital hybrid mode power amplifiersystem implemented with a down converter (DNC) and UPC-based clippingerror restoration path according to another embodiment of the presentinvention.

FIG. 8 is a block diagram showing a digital hybrid mode power amplifiersystem implemented with a DNC and an analog quadrature modulator (AQM)according to another embodiment of the present invention.

FIG. 9 is a block diagram showing an embodiment of the analog quadraturemodulator compensation structure.

GLOSSARY

The acronyms used herein have the following meanings:

ACLR Adjacent Channel Leakage Ratio ACPR Adjacent Channel Power RatioADC Analog to Digital Converter AQDM Analog Quadrature Demodulator AQMAnalog Quadrature Modulator AQDMC Analog Quadrature DemodulatorCorrector AQMC Analog Quadrature Modulator Corrector BPF Bandpass FilterCDMA Code Division Multiple Access CFR Crest Factor Reduction DACDigital to Analog Converter DET Detector DHMPA Digital Hybrid Mode PowerAmplifier DDC Digital Down Converter DNC Down Converter DPA DohertyPower Amplifier DQDM Digital Quadrature Demodulator DQM DigitalQuadrature Modulator DSP Digital Signal Processing DUC Digital UpConverter EER Envelope Elimination and Restoration EF Envelope FollowingET Envelope Tracking EVM Error Vector Magnitude FFLPA Feedforward LinearPower Amplifier FIR Finite Impulse Response FPGA Field-Programmable GateArray

GSM Global System for Mobile communications

I-Q In-phase/Quadrature IF Intermediate Frequency

LILAC Linear Amplification using Nonlinear Components

LO Local Oscillator LPF Low Pass Filter MCPA Multi-Carrier PowerAmplifier MDS Multi-Directional Search OFDM Orthogonal FrequencyDivision Multiplexing PA Power Amplifier PAPR Peak-to-Average PowerRatio PD Digital Baseband Predistortion PLL Phase Locked Loop QAMQuadrature Amplitude Modulation QPSK Quadrature Phase Shift Keying RFRadio Frequency SAW Surface Acoustic Wave Filter UMTS Universal MobileTelecommunications System UPC Up Converter WCDMA Wideband Code DivisionMultiple Access WLAN Wireless Local Area Network DETAILED DESCRIPTION OFTHE INVENTION

The present invention is a novel RF-out PA system that utilizes anadaptive digital predistortion algorithm. The present invention is ahybrid system of digital and analog modules. The interplay of thedigital and analog modules of the hybrid system both linearize thespectral regrowth and enhance the power efficiency of the PA whilemaintaining or increasing the wide bandwidth. The present invention,therefore, achieves higher efficiency and higher linearity for widebandcomplex modulation carriers.

FIG. 1 is a high level block diagram showing the basic systemarchitecture which can be thought of, at least for some embodiments, ascomprising digital and analog modules and a feedback path. The digitalmodule is the digital predistortion controller 101 which comprises thePD algorithm, other auxiliary DSP algorithms, and related digitalcircuitries. The analog module is the main power amplifier 102, otherauxiliary analog circuitries such as DPA, and related peripheral analogcircuitries of the overall system. The present invention is a “blackbox”, plug-and-play type system because it accepts RF modulated signal100 as its input, and provides a substantially identical but amplifiedRF signal 103 as its output, therefore, it is RF-in/RF-out. Basebandinput signals can be applied directly to the Digital PredistorterController according to one embodiment of the present invention. AnOptical input signal can be applied directly to the Digital PredistorterController according to one embodiment of the present invention. Thefeedback path essentially provides a representation of the output signalto the predistortion controller 101. The present invention is sometimesreferred to as a digital hybrid mode power-amplifier (DHMPA) systemhereinafter.

Digital Predistorter Algorithm

Digital Predistortion (DPD) is a technique to linearize a poweramplifier (PA). FIG. 2 shows in block diagram form an embodiment of alinear digitally predistorted PA. In the DPD block, a memory polynomialmodel is used as the predistortion function (FIG. 3), and complies withthe formula:

  z(n) = ??(?)(??x_(i)(n − i)?)?indicates text missing or illegible when filed

where a_(ij) are the DPD coefficients.

In the DPD estimator block, a least square algorithm is utilized to findthe DPD coefficients, and then transfer them to DPD block. The detailedDPD algorithm is shown in FIG. 4.

FIG. 3 is a block diagram showing a predistortion (PD) part in the DHMPAsystem of the present invention. The PD in the present inventiongenerally utilizes an adaptive polynomial-based digital predistortionsystem. Another embodiment of the PD utilizes a LUT-based digitalpredistortion system. More specifically, the PD illustrated in FIG. 3and in embodiments disclosed in FIG. 7 and FIG. 8, discussed below, areprocessed in the digital processor by an adaptive algorithm, presentedin U.S. patent application Ser. No. 11/961,969, entitled A Method forBaseband Predistortion Linearization in Multi-Channel WidebandCommunication Systems. The PD for the DHMPA system in FIG. 3 hasmultiple finite impulse response (FIR) filters, that is, FIR1 301, FIR2303, F1R3 305, and FIR4 307. The PD also contains the third orderproduct generation block 302, the fifth order product generation block304, and the seventh order product generation block 306. The outputsignals from FIR filters are combined in the summation block 308.Coefficients for multiple FIR filters are updated by the digitalpredistortion algorithm.

Delay Estimation Algorithm:

The DPD estimator compares x(n) and its corresponding feedback signaly(n-Ad) to find the DPD coefficients, where Ad is the delay of thefeedback path. As the feedback path delay is different for each PA, thisdelay should be identified before the signal arrives at the coefficientestimation. In this design, the amplitude difference correlationfunction of the transmission, x(n), and feedback data, y(n), is appliedto find the feedback path delay. The correlation is given by

  c(m) = ?sign(x(i + 1) − x(i))sign(y(i + m + 1) − y(i + m))  n(delay) = Max(C(m))?indicates text missing or illegible when filed

The delay n that maximizes the correlation C(m) is the feedback pathdelay. The delay estimation block is shown in FIG. 5.

Since the feedback path goes through analog circuitry, the delay betweenthe transmission and feedback path could be a fractional sample delay.To synchronize the signals more accurately, fractional delay estimationis necessary. To simplify the design, only a half-sample delay isconsidered in this design, as shown in FIG. 6. It will be appreciatedthat smaller fractional delays can also be utilized in at least someembodiments.

To get the half-sample delay data, an upsampling approach is the commonchoice, but in this design, in order to avoid a very high samplingfrequency in the FPGA, an interpolation method is used to get thehalf-sample delay data. The data with integer delay and fractional delayare transferred in parallel. The interpolation function for fractionaldelay is

${y(n)} = {\sum\limits_{i = 0}^{3}\; {c_{i}{x\left( {n + i} \right)}}}$

in which c_(i) is the weight coefficient.

Whether the fractional delay path or the integer delay path will bechosen is decided by the result of the amplitude difference correlator.If the correlation result is odd, the integer path will be chosen,otherwise the fractional delay path will be chosen.

Phase Offset Estimation and Correction Algorithm:

Phase offset between the transmission signal and the feedback signalexists in the circuit. For a better and faster convergence of the DPDcoefficient estimation, this phase offset should be removed.

The transmission signal x(n) and feedback signal y(n) can be expressedas

x(n)=|x(n)|e ^(jθx) and y(n)=|y(n)|e ^(jθy),

The phase offset e^(j(θx-θy)) can be calculated through

$\text{?} = \frac{{x(n)}{y(n)}^{*}}{{{x(n)}}{{y(n)}}}$?indicates text missing or illegible when filed

So, the phase offset between the transmission and feedback paths is

$\text{?} = {{mean}\left( \frac{{x(n)}{y(n)}^{*}}{{{x(n)}}{{y(n)}}} \right)}$?indicates text missing or illegible when filed

The feedback signal with the phase offset removed can be calculated by

$\mspace{20mu} {{\overset{\_}{y}(n)} = {{y(n)}\text{?}}}$?indicates text missing or illegible when filed

Magnitude Correction:

As the gain of the PA may change slightly, the feedback gain should becorrected to avoid the error from the gain mismatch. The feedback signalis corrected according to the function

${\overset{\_}{y}(n)} = {{y(n)}\frac{\sum\limits_{i = 1}^{N}\; {{x(i)}}}{\sum\limits_{i = 1}^{N}\; {{y(i)}}}}$

In this design, N is chosen to be 4096. The choice of N will depend onthe desired accuracy.

QR_RLS Adaptive Algorithm:

The least square solution for DPD coefficient estimation is formulatedas

  F(x(n)) = y(n)$\mspace{20mu} {{F\left( {x(n)} \right)} = {\sum\limits_{i = 1}^{N}\; {\sum\limits_{j = 0}^{K}\; {a_{ij}{x\left( {n - i} \right)}{{x\left( {n - i} \right)}}\text{?}}}}}$?indicates text missing or illegible when filed

Define h_(k)−x(n−i)|^(j), w_(k)−a_(ij), where k−(I−1)N+j. The leastsquare formulation can be expressed as:

?w_(k)h_(k) = y(n) ?indicates text missing or illegible when filed

In this design, QR-RLS algorithm (Haykin, 1996) is implemented to solvethis problem. The formulas of QR_RLS algorithm are

$\quad\mspace{79mu} \left\{ {\begin{matrix}{{d(i)}\overset{\bigtriangleup}{=}{{y(i)} - {h_{i}\overset{\_}{w}}}} \\{{\overset{\_}{w}}_{i}\overset{\bigtriangleup}{=}{w_{i} - \overset{\_}{w}}} \\{q_{i}\overset{\bigtriangleup}{=}{\text{?}\left\lbrack {w_{i} - \overset{\_}{w}} \right\rbrack}}\end{matrix}\text{?}\text{indicates text missing or illegible when filed}} \right.$

where Φ_(i) is a diagonal matrix, and q_(i) is a vector.The QR_RLS algorithm gets the ith moment (I); and qi from its (i−1)thmoment through a unitary transformation:

$\begin{matrix}{\mspace{79mu} {{A = {\begin{bmatrix}\phi_{i}^{1/2} & 0 \\q_{i}^{*} & {{_{a}^{*}(i)}\text{?}(i)} \\\text{?} & \text{?}\end{bmatrix} = {\begin{bmatrix}\text{?} & \text{?} \\\text{?} & {d(i)}^{*} \\0 & 1\end{bmatrix}\text{?}}}}{\text{?}\text{indicates text missing or illegible when filed}}}} & \;\end{matrix}$

-   -   θ_(i) is a unitary matrix for unitary transformation.

To apply QR_RLS algorithm more efficiently in FPGA, a squared-root-freeGivens rotation is applied for the unitary transformation process (E. N.Frantzeskakis, 1994)

$\mspace{20mu} {\begin{bmatrix}a_{1} & a_{2} & \ldots & a_{n} \\b_{1} & b_{2} & \ldots & b_{n}\end{bmatrix} = {{{{\begin{bmatrix}\sqrt{k_{a}} & 0 \\0 & \sqrt{k_{b}}\end{bmatrix}\begin{bmatrix}a_{1}^{\prime} & a_{2}^{\prime} & \ldots & a_{n}^{\prime} \\b_{1}^{\prime} & b_{2}^{\prime} & \ldots & b_{n}^{\prime}\end{bmatrix}}\mspace{20mu}\begin{bmatrix}a_{1}^{\prime} & a_{2}^{\prime} & \ldots & a_{n}^{\prime} \\b_{1}^{\prime} & b_{2}^{\prime} & \ldots & b_{n}^{\prime}\end{bmatrix}}\theta} = {\begin{bmatrix}\sqrt{k_{a}^{\prime}} & 0 \\0 & \sqrt{k_{b}^{\prime}}\end{bmatrix}\begin{bmatrix}1 & a_{2}^{''} & \ldots & a_{n}^{''} \\0 & b_{2}^{''} & \ldots & b_{n}^{''}\end{bmatrix}}}}$   k_(a)^(′) = k_(a)a₁² + k_(b)b₁²  k_(b)^(′) = k_(a)k_(b)/k_(a)^(′)  ? = (k_(a)a₁? + k_(b)b₁?)/k_(a)^(′)   ? = −b₁? + a₁??indicates text missing or illegible when filed

For RLS algorithm, the ith moment is achieved as below:

$\mspace{79mu} {{\begin{bmatrix}\text{?} & h_{i}^{*} \\\text{?} & \text{?} \\0 & 1\end{bmatrix}\theta_{i}} = {\begin{bmatrix}\text{?} & 0 \\\text{?} & \text{?} \\\text{?} & \text{?}\end{bmatrix}\begin{bmatrix}\sqrt{k_{a}^{\prime}} & 0 \\0 & \sqrt{k_{b}^{\prime}}\end{bmatrix}}}$ ?indicates text missing or illegible when filed

w_(i) can be obtained by solving

Φ{dot over (2)}[w _(i) − w]= q _(i)

In the iterative process, a block of data (in this design, there are4096 data in one block) is stored in memory, and the algorithm uses allthe data in memory to estimate the DPD coefficient. In order to make theDPD performance more stable, the DPD coefficients are only updated afterone block of data is processed. The matrix A will be used for the nextiteration process, which will make the convergence faster.

To make sure the performance of the DPD is stable, a weighting factor fis used when updating the DPD coefficients as

w _(i) =f×w _(i−1)+(1−f)w _(i)

The DPD coefficient estimator calculates coefficients w_(i) by usingQR_RLS algorithm. These w_(i) are copied to the DPD block to linearizethe PA.

FIGS. 7 and 8 are block diagrams showing more sophisticated embodimentsof DHMPA system, where like elements are indicated with like numeralsand elements not numbered in FIG. 8 have the same reference numerals asshown in FIG. 7. The embodiments of FIGS. 7 and 8 apply crest factorreduction (CFR) prior to the PD with an adaptation algorithm in onedigital processor, so as to reduce the PAPR, EVM and ACPR and compensatethe memory effects and variation of the linearity due to the temperaturechanging of the PA. The digital processor can take nearly any form; forconvenience, an FPGA implementation is shown as an example, but ageneral purpose processor is also acceptable in many embodiments. TheCFR implemented in the digital module of the embodiments is based on thescaled iterative pulse cancellation presented in patent application U.S.61/041,164, filed Mar. 31, 2008, entitled An Efficient Peak CancellationMethod For Reducing The Peak-To-Average Power Ratio In WidebandCommunication Systems, incorporated herein by reference. The CFR isincluded to enhance performance and hence optional. The CFR can beremoved from the embodiments without affecting the overallfunctionality.

FIG. 7 is a block diagram showing a DHMPA system implemented with DQM inaccordance with an embodiment of the present invention. The system shownin FIG. 7 has a dual mode of RF-in 700 and/or multi-carrier digitalsignal 705 at the input, and an RF signal at the output 710. The dualmode of signal input allows maximum flexibility: RF-in (the “RF-inMode”) or baseband digital-in (the “Baseband-in Mode”). The system shownin FIG. 7 comprises three key portions: a reconfigurable digital(hereinafter referred as “FPGA-based Digital”) module 715, a poweramplifier module 760 and a feedback path 725.

The FPGA-based Digital part comprises a digital processor 715 (e.g.FPGA), digital-to-analog converters 735 (DACs), analog-to-digitalconverters 740 (ADCs), and a phase-locked loop (PLL) 745. Since theembodiment of FIG. 7 has a dual input mode, the digital processor hastwo paths of signal processing. For the RF signal input path, thedigital processor has implemented a digital quadrature demodulator(DQDM), a CFR, a PD, and a digital quadrature modulator (DQM). For thebaseband digital input path, a digital up-converter (DUC), CFR, PD, anda DQM are implemented.

The RF-in Mode of the embodiment shown in FIG. 7 has implemented a downconverter (DNC) 750 prior to the FPGA-based Digital part and an ADC 740prior to the FPGA. An analog down converted signal is provided to theFPGA-based Digital module and converted to a digital signal by the ADC740. The digitally converted signal is demodulated by the DQDM togenerate both real and imaginary signals and then PAPR of the signal isreduced by CFR. The peak reduced signal is predistorted to linearize theamplifier and is passed through a DQM to generate the real signal andthen converted to an intermediate frequency (IF) analog signal by a DACin the FPGA-based Digital part. However, it is not required in allembodiments to implement DQDM and DQM in the FPGA. If, as shown in FIGS.7 and 8, a digital modulator will not be used, then two DAC's 801 behindthe FPGA feeding AQM module 800 can be used to generate real andimaginary signals, respectively (the “AQM Implementation”).

The Baseband-in Mode of the system of FIG. 7 works slightly differentlyfrom the RF-in Mode. Digital data streams from multi-channels as I-Qsignals are coming to the FPGA-based Digital module and are digitallyup-converted to digital IF signals by the DUC. From this point onwards,the Baseband-in Mode and RF-in Mode proceeds identically. These IFsignals are then passed through the CFR block so as to reduce thesignal's PAPR. This PAPR suppressed signal is digitally predistorted inorder to pre-compensate for nonlinear distortions of the poweramplifier.

In either input mode, the memory effects due to self-heating, biasnetworks, and frequency dependencies of the active device arecompensated by the adaptation algorithm in the PD, as well. Thecoefficients of the PD are adapted by a synchronizing the widebandcaptured output signal from the feedback path 725 with the referencesignal. The digital predistortion algorithm performs the synchronizationand compensation. The predistorted signal is passed through a DQM inorder to generate the real signal and then converted to an IF analogsignal by the DAC 740 as shown. As disclosed above, the DQM is notrequired to be implemented in the FPGA, or at all, in all embodiments.Alternatively, if the DQM is not used in the FPGA, then the AQMImplementation can be implemented with two DACs to generate real andimaginary signals, respectively. The gate bias voltage 753 of the poweramplifier is determined by the adaptation algorithm and then adjustedthrough the DACs 535 in order to stabilize the linearity fluctuationsdue to the temperature changes in the power amplifier.

The power amplifier part comprises a UPC for a real signal (such asillustrated in the embodiment shown in FIG. 7), or an AQM for real andcomplex signals (such as the embodiment of a DHMPA system depicted inFIG. 8) from the FPGA-based Digital module, a high power amplifier withmulti-stage drive amplifiers, and a temperature sensor. In order toimprove the efficiency performance of the DHMPA system, efficiencyboosting techniques such as Doherty, Envelope Elimination andRestoration (EER), Envelope Tracking (ET), Envelope Following (EF), andLinear amplification using Nonlinear Components (LINC) can be used,depending upon the embodiment. These power efficiency techniques can bemixed and matched and are optional features to the fundamental DHMPAsystem. One such Doherty power amplifier technique is presented incommonly assigned U.S. Provisional Patent Application Ser. No.60/925,577, filed Apr. 23, 2007, entitled N-Way Doherty DistributedPower Amplifier, incorporated herein by reference, together with U.S.patent application Ser. No. 12/603,419, filed Oct. 21, 2009, entitledN-Way Doherty Distributed Power Amplifier with Power Tracking. Tostabilize the linearity performance of the amplifier, the temperature ofthe amplifier is monitored by the temperature sensor and then the gatebias of the amplifier is controlled by the FPGA-based Digital part.

The feedback portion comprises a directional coupler, a mixer, a gainamplifier, a band pass filter (BPF), and a Digital to Analog Converter(DAC). Depending upon the embodiment, these analog components can bemixed and matched with other analog components. Part of the RF outputsignal of the amplifier is sampled by the directional coupler and thendown converted to an IF analog signal by the local oscillation signal inthe mixer. The IF analog signal is passing through the gain amplifier,and the BPF (e.g., surface acoustic wave filter) which can capture theout-of-band distortions. The output of the BPF is provided to the ADC ofthe FPGA-based Digital module in order to determine the dynamicparameters of the digital PD depending on output power levels andasymmetrical distortions due to the memory effects. In addition,temperature is also detected by the detector 580 to calculate thevariation of linearity and then adjust gate bias voltage of the PA. Moredetails of the PD algorithm and self-adaptation feedback algorithm canbe appreciated from FIG. 3, discussed above, which shows apolynomial-based predistortion algorithm and from FIG. 4, also discussedabove, which shows the block diagram of the digital predistortersynchronization algorithm which can be used in some embodiments of theinvention.

In the case of a strict EVM requirement for broadband wireless accesssuch as WiMAX or other OFDM based schemes (EVM<2.5%), the CFR in theFPGA-based Digital part is only able to achieve a small reduction of thePAPR in order to meet the strict EVM specification. In generalcircumstances, this means the CFR's power efficiency enhancementcapability is limited. In some embodiments of the present invention, anovel technique is included to compensate the in-band distortions fromCFR by use of a “Clipping Error Restoration Path” 790, hence maximizingthe DHMPA system power efficiency in those strict EVM environments. Asnoted above, the Clipping Error Restoration Path has an additional DAC735 in the FPGA-based Digital portion and an extra UPC 720 in the poweramplifier part (see FIGS. 7 and 8). The Clipping Error Restoration Pathcan allow compensation of in-band distortions resulting from the CFR atthe output of the power amplifier. Further, any delay mismatch betweenthe main path and the Clipping Error Restoration Path can be alignedusing digital delay in the FPGA.

Referring again to FIG. 7, the RF input signal is first down-convertedto baseband digital signals, and then digitally up-converted to digitalIF signals (−7.5 MHz, −2.5 MHz, 2.5 MHz, 7.5 MHz). If the system of FIG.7 has a Baseband-in Mode, then the digital data streams frommulti-channels are digitally up-converted to digital IF signals (−7.5MHz, −2.5 MHz, 2.5 MHz, 7.5 MHz) directly as they enter the digitalprocessor. The CFR then reduces the PAPR. The peak-reduced signal ispredistorted to linearize the DPA and is passing through two DACs forreal and imaginary signals and finally through an AQM.

FIG. 9. is a block diagram showing an embodiment of the analogquadrature modulator compensation structure. The input signal isseparated into an in-phase component X_(i) and a quadrature componentX_(Q). The analog quadrature modulator compensation structure comprisesfour real filters {g11, g12, g21, g22} and two DC offset compensationparameters c1, c2. The DC offsets in the AQM will be compensated by theparameters c1, c2. The frequency dependence of the AQM will becompensated by the filters {g11, g12, g21, g22}. The order of the realfilters is dependent on the level of compensation required. The outputsignals YI and YQ will be presented to the AQM's in-phase and quadratureports.

The configuration of the power amplifier part and the feedback part ofthe system shown in FIG. 8 are the same as for the system shown in FIG.7.

In the system shown in FIG. 8, the DNC frequency translates the RFsignal into a low IF signal. The IF signal is then presented to the ADCwhereupon it is digitally down-converted to baseband followed by CFR andpredistortion (PD). The output of the PD is a baseband signal which willthen be digitally upconverted to an IF frequency and presented to theDAC. The output of the DAC is then further frequency translated to a RFfrequency through the up-converter (UPC.) The configuration of the poweramplifier part and the feedback part of system of FIG. 8 are the same asfor the FIG. 7 System.

In summary, the DHMPA system of the present invention enhancesefficiency and linearity relative to the prior art since the DHMPAsystem is able to implement CFR, DPD and adaptation algorithms in onedigital processor, which consequently saves hardware resources andprocessing time. The DHMPA system is also reconfigurable andfield-programmable since the algorithms and power-efficiency-enhancingfeatures can be adjusted like software in the digital processor atanytime.

Furthermore, since the DHMPA system accepts RF modulated signal asinput, it is not necessary to use coded I and Q channel signals in thebaseband. Therefore, the performance of wireless base-station systemscan be enhanced simply by replacing the existing PA modules with theDHMPA. The result is that the present invention provides a “plug andplay” PA system solution such that the structure of existingbase-station systems does not need to be modified or rebuilt for a newset of signal channels in order to benefit from high efficiency and highlinearity PA system performance.

Moreover, the DHMPA system is agnostic to modulation schemes such asquadrature phase shift keying (QPSK), quadrature amplitude modulation(QAM), Orthogonal Frequency Division Multiplexing (OFDM), etc. in codedivision multiple access (CDMA), global system for wirelesscommunications (GSM), WCDMA, CDMA2000, and wireless LAN systems. Thismeans that the DHMPA system is capable of supporting multi-modulationschemes, multi-carriers and multi-channels. Other benefits of the DHMPAsystem of the present invention include correction of PA non-linearitiesin repeater or indoor coverage systems that do not have the necessarybaseband signals information readily available.

Although the present invention has been described with reference to thepreferred embodiments, it will be understood that the invention is notlimited to the details described thereof. Various substitutions andmodifications have been suggested in the foregoing description, andothers will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the invention as defined in the appended claims.

We claim:
 1. A digital predistortion system for linearizing the outputof power amplifiers comprising input signal for wireless communications,at least one power amplifier for outputting an amplified signal, atleast one feedback signal derived from the amplified signal including arepresentation of a noise characteristic of the power amplifier,estimator logic responsive to the at least one feedback signal fordetermining predistortion coefficients at least partly in accordancewith the equation  z(n) = ??(?)(??x_(i)(n − i)?), ?indicates text missing or illegible when filedwhere z(n) is a predistortion function, n is a variable representingtime, x_(t) is a transmission (input) signal, i, j, and k are countersfor summation, N is a number of data per block, and, a_(ij) arepredistortion coefficients, and digital predistortion logic forpredistorting the input signal and supplying a predistorted signal tothe at least one power amplifier.
 2. The digital predistortion system ofclaim 1 further comprising a digital module that includes one or more ofa digital field programmable gate array, digital-to-analog converters,analog-to-digital converters, and a phase-locked loop.
 3. The digitalpredistortion system of claim 1 wherein the power amplifier comprises atleast one of an up-converter for real signals and an analog quadraturemodulator for real and complex signals, a high power amplifier withmulti-stage drive amplifiers, and a temperature sensor.